1. Field of the Invention
This invention relates to a method of lap welding an eventual can blank consisting of a metal sheet or the like and, more particularly, a method noted above, in which a lap joint of an eventual can blank consisting of a metal sheet or a covered metal sheet such as a plated metal sheet in a cylindrical form is bonded by heating it and also simultaneously pressing it by passing it between upper and lower roller electrodes via upper and lower wire electrodes.
2. Description of the Prior Art
Heretofore, seam welding has been well known as a resistance welding process, and it is also referred to as joint welding. By this welding process, since a weld zone having an excellent seal property can be obtained, it is used for making liquid transport pipes, fuel tanks, drum cans, food cans, etc., and various seam welding processes have been proposed depending on the kind of weld joint and characters of materials such as steel plates or sheets to be welded. However, there is no welding process proposed, which can be used for manufacturing food cans and drink cans only.
Lap seam welding is the most general process among various seam welding processes, in which the lap joint consists of overlapped parts of a steel sheet and is welded by energizing it under pressure in a state pinched between upper and lower disk-like or roller electrodes. This welding process can be regarded as continuous repetition, caused by the roller electrodes, of spot welding which is the most popular resistance welding process. The lap joint is energized intermittently and regularly during welding process, and the weld zone thereof is thermally fused by the Joule heat, so that circular nuggets are formed continuously. Thus, in the lap seam joint, the overlapped upper and lower steel plates are seamed and jointed together by the continuous nuggets. A weld zone having excellent air-tightness and gas-tightness thus can be obtained. The process is therefore also referred to as seam welding from this bonded state.
In the lap seam welding, however, the width of the contact surfaces of the upper and lower roller electrodes is smaller than the width of the lap joint, i.e., overlapped parts. Therefore, bondless portions will remain on the opposite side of the weld line in the weld zone. When the weld zone is subsequently coated with a synthetic resin, the opposite edges are coated with difficulty. Corrosion, therefore, proceeds from these bondless portions, and there is a possibility of cutting a hand when handling the welded product. Accordingly, mash seam welding, which is a variety of the lap seam welding, has been proposed as a welding process, with which bondless portions will not remain on the opposite sides of the weld line.
In the mash seam welding process, while the lap joint is fused under a pressure it is mashed between upper and lower roller electrodes having contact surfaces of a width greater than the width of the lap joint, whereby a seam weld is obtained. Unlike the ordinary lap seam welding process, the welding surfaces in the weld zone are mashed obliquely to be fused and press bonded. In this case, no bondless portions will remain on the opposite sides of the weld line. In the weld zone, therefore, nuggets are formed continuously due to fusion, and the welding surfaces are press bonded together. Hence, the weld has high mechanical strength and are excellent seal property.
Meanwhile, drink cans and food cans are rarely manufactured from cold rolled steel sheet. Instead, covered steel sheets, i.e., tin-plated steel sheet, are mostly used to meet anti-corrosion demand. When the covered steel sheet is seam welded using the roller electrodes noted above, the covering metal, e.g., tin, migrates to and contaminates the periphery, i.e., contact surface, of the roller electrodes. Accordingly, wire electrodes such as copper wires are provided on the outer periphery of the roller electrodes so that the covered steel sheet is in contact with the wire electrodes to effect the seam welding. One such method is disclosed in Japanese Patent Publication No. 25213/69.
In this welding process, circumferential grooves are formed in the outer periphery of the upper and lower roller electrodes, and wire electrodes are received in the grooves to be fed with the rotation of the roller electrodes, whereby the lap joint is fused while it is pressed between the upper and lower wire electrodes to form nuggets and effect welding. Again in this process employing the wire electrodes, like the case of the solitary roller electrodes noted above, the problem of bondless portions remaining on the opposite sides of the weld line will arise if the width of the contact surfaces of the wire electrodes is smaller than the width of the lap joint. Accordingly, it has been proposed to preliminarily flatten the wire electrode into an oval sectional shape to make the width of the contact surface greater than the width of the lap joint for mash seam welding to produce drink cans and food cans. Such a mash seam welding process is discussed in Japanese Patent Publication No. 26213/79.
In this disclosed mash seam welding process, in addition to making the width of the contact surface of wire electrode greater than the width of the lap joint, the width of the lap joint is set to 3 to 6 times the thickness of the steel sheet blank. Although in this process the width of the lap joint is restricted in relation to the thickness of the sheet blank, actually there is no definite technical basis of the relation between the thickness of the sheet blank and the width of the lap joint. The relation is set from mere consideration of the fact that thin steel sheets as thick as about 0.15 to 0.25 mm are used for drink cans and food cans. More specifically, even if the lap joint is formed from overlapped edges of a steel sheet blank in a cylindrical form using a guide bar of Z-shaped sectional profile as disclosed in Japanese Patent Publication No. 31449/69, the actualized limit of the width thereof is approximately 0.8 mm. Essentially, the mash seam welding is carried out using wire electrodes having a couple of time the width of the lap joint. In a further aspect, tin-plated steel sheet that are actually used for the manufacture of drink cans and food cans, have a resin coating preliminarily printed on their surface, which will be changed during the fixing of the resin coating. The fact, however, is not taken into consideration. Furthermore, the welded eventual can blank is subjected for flange formation and rounding processes for attaching the bottom and lid to complete a can, but a weld zone which has a mechanical strength sufficient to withstand these processes can not be obtained by welding process noted above. The foregoing is summarized as follows.
(1) The blank for the drink cans and food cans is cut into a required size from a steel sheet. On the surface of the blank is printed, except for edge portions for forming the lap joint, predetermined characters and drawings in the form of a resin coating. After the coating film is fixed, the blank is shaped into a cylindrical form and then the lap joint is welded. However, the surface character of the edge portions free from coating is changed by the heat generated during the fixing of the coat. For example, tin of a plating layer is considerably alloyed with iron. This leads to considerable fluctuations of the contact resistance at the time of the welding. When welding is performed under this condition, such welding defects as scatering, gapping and reaking are liable to be produced in the weld zone due to the character change noted above.
(2) Where the width of the lap joint is 3 to 6 times the thickness of the blank and the blank is as thin as 0.2 mm, for instance, the width of the lap joint is as large as 0.8 mm. In this case, therefore, it is necessary to cause as much current as possible at the time of the welding and hence the proper current range is narrow, with which an erroneous current control would result in increased fusion. In such a case, a weld zone capable of withstanding the end flange formation process after welding cannot be obtained. In addition, with an increase of the width of the lap joint the diameter of the wire electrode has to be increased, which is a great economical disadvantage because an expensive copper wire is consumed as a wire electrode in a single welding operation. The increase of the width of the lap joint also increases the width of the overlapped edge portions of the blank material, e.g., steel sheet, which is undesired from the standpoint of economy.
(3) Since the width of the contact surface of the wire electrode is smaller compared to the width of the lap joint, typically a couple of times the width of the lap joint, it is necessary to strictly regulate the position of the lap joint with respect to the upper and lower wire electrodes. This regulation is very difficult and is liable to be lost due to such cause as vibrations of the welder. Once the regulation is lost, the welding property is extremely deteriorated to extremely reduce the yield.
(4) When forming nuggets to produce a lap joint in mash seam welding, the weld zone has to be partly fused and solidified. The nugget has a different structure from the ordinary casting structure. Due to the partial fusion of the weld zone, the hardness of the nuggents is increased compared to the rest of the weld zone and the structure thereof is also changed. Therefore, the weld zone is readily cracked during double binding or flange formation processes after the welding process due to the difference in the hardness and structure.